Since the light absorption of cyanine dyes shifts bathochromically as the dye chromophore is lengthened, monomethine cyanine dyes, characterized by a single methine linking the dye nuclei, are particularly useful in absorbing blue light. Benzimidazolo monomethine cyanine dyes, precursors thereof, and methods for their preparation are known in the art, as illustrated by Ferrania U.K. Patent No. 1,096,104 and Bader et al U.S. Pat. No. 3,337,578. When benzimidazolo cyanine dyes are employed as sensitizers in silver halide emulsions, it is known that the presence of one or more electron-withdrawing groups reduces the basicity of the dyes and increases their sensitizing efficiency. It is also known that the relative orientation of dye nuclei can affect the manner and efficiency with which they are adsorbed to silver halide grain surfaces and therefore significantly influence their sensitizing characteristics.
One technique that can be employed for orienting benzimidazolo monomethine cyanine dye nuclei is to employ an alkanediyl (alkylene) group bridging the benzimidazolo nuclei. Brooker U.S. Pat. No. 2,479,152 discloses 1,1'-dialkyl-3,3'-alkylene-benzimidazolo monomethine cyanine dyes and a process for their synthesis. Unfortunately the synthetic process taught by Brooker will not produce alkanediyl bridged benzimidazolo monomethine cyanine dyes including commonly employed electron-withdrawing substituents on the benzimidazolo nuclei. Thus, although alkanediyl bridging is capable of relatively orienting the nuclei for efficient adsorption to silver halide grain surfaces, the sensitizing action of the dyes is not as highly efficient because of their high basicity, as compared to the preferred, electron-withdrawing group substituted benzimidazolo cyanine dyes employed as spectral sensitizers. Further, the synthetic process of Brooker effectively restricts the quaternizing 1,1' substituents to alkyl. (Although Brooker assigns quaternizing alkyl substituents 1,1' ring designations and the alkanediyl bridging group 3,3' ring designations, it should be noted that these ring position designations are reversed when the alkanediyl bridging group precedes the quaternizing substituent in alphabetical order.)